1. Field of the Invention
This invention relates generally to polyesters and polyester containers and particularly to polyesters and polyester containers having a reduced coefficient of friction and improved clarity.
2. Description of the Prior Art
Problems exist in conveying various types of polyester containers due to the excessive amount of static friction encountered when container surfaces contact. This excessive friction can lead to “process line” or “filling line” interruptions that are economically undesirable. The problem occurs after the polyester polymer has been molded into preforms or stretch blown into various types of containers. The containers are sometimes conveyed directly into a palletizing station and then shipped to a filling plant or they are conveyed to a labeling and filling line contained within the same plant. This problem is more pronounced in the carbonated soft-drink (“CSD”) industry due to the high speed of stretch-blow molding conveying and filling lines. The problem is also encountered in other parts of the polyester container industry where the containers are being conveyed under pressures applied from congested areas of the conveying process.
During the process of blow molding or injection molding container preforms, the preforms are fed into a large box (gaylord box) that holds more than 1000 preforms. Given the high coefficient of friction (“COF”) that is common between polyester surfaces, the preforms tend to stack on top of one another in a conical shape as viewed from the side of the box. This stacking results in fewer preforms being loaded into a box and therefore higher shipping costs per preform. The high level of friction between the preform surfaces can also cause jams in the feeder bin as the preforms are loaded onto the feed rail. Similarly, jams may also occur on the feed rail due to such friction.
Straight-walled containers such as the two liter bottles used in the carbonated soft-drink (“CSD”) industry have a very smooth surface that maximizes the amount of surface area that comes in contact between two adjacent bottles. With the inherently high COF of polyester containers such as PET (PET has a static COF greater than 1.0), the containers become entangled and “tip over” or just stop moving in the conveying line after blowing or during filling. Such tip over and stopage obviously causes undesirable disruptions in the conveying or filling process.
A high COF prevents adjacent containers on a multiple-row conveying line from moving (turning or slipping) during conveying. When the conveying line changes direction, sometimes as much as 90 degrees, the containers may become entangled and either stay upright and stop the feed or tip over and stop the line. In either event, someone has to monitor these problem areas at all times to keep the line moving. Therefore, a container having a low static COF that could slide and rotate against other containers during conveying would minimize or eliminate process downtime and the need for someone to constantly monitor the process. These problems are all related to polyesters having an unacceptably COF.
There is prior art relating to methods for reducing the COF for polyesters. One such method involves the addition of an antiblock agents such as silica, talc, calcium carbonate, calcium stearate, and other inorganic compounds. JP 9272191 discloses a multi-layer sheet containing inert particulates (10 to 5000 ppm at 0.5 to 30 micron size) including silica and talc used to improve the slip properties, scratch resistance, cut properties and adhesive properties of the sheet and articles made from the sheet. U.S. Pat. No. 5,840,419 discloses multilayer polyolefin films that use cross-linked silicone in combination with inorganic antiblock agents such as talc in amounts of 500 to 5000 ppm with particles sizes from 1 to 6 micrometers to reduce the COF. Neither reference discloses stretch blow molded containers. U.S. Pat. No. 6,323,271 discloses polyester resins containing a silica selected from the group consisting of fumed silica, colloidal silica and silica beads that is useful for making containers having reduced stickiness relative to containers made from the same resin but without the silica. U.S. Pat. No. 5,258,161 discloses polyolefin films having talc in amounts of 0.05% to 3% by weight as an antiblock agent. U.S. Pat. No. 5,908,890 disclose a polymer film comprising a polyolefin matrix containing a pumice antiblock agent in amounts of less than about 1 percent by weight.
U.S. Pat. No. 5,830,544 discloses poly(ethyleneterephthalate) (“PET”) bottles having reduced stickiness due to the addition of amorphous silica at a concentration range of 10 to 100 ppm. The use of additives other than amorphous silica and methods for improving clarity of the bottle containing anti-stick additives are not disclosed. JP Patent Application Heisei 2-307117 discloses the optimization of loading and particulate size of the antiblock on film properties such as haze and COF. The reference does not disclose containers such as plastic bottles nor does it disclose drying an antiblock before processing.
JP Patent Application Heisei 4-180957 discloses mono- and multi-layer sheet and thermoformed moldings formed from PET having 100 to 10,000 ppm talc with particle size less than 10 microns. Suitable thermoformed containers include blister packs that have good clarity. Containers such as plastic bottles, the benefits of drying the antiblock before processing, and the use of fatty acid tethers are not disclosed.
There is prior art relating to polyester films incorporating a variety of inorganic particles to improve crystallinity and slip. JP 7238211 discloses magnetic tape; JP 6065478 discloses magnetic tape, photograph, packaging film; JP5104621 discloses thermoforned sheets; JP 4183718 discloses base film for magnetic tape, photographic film, electrical insulating film, a base material for gold yarn, and packaging material; and JP 4180957 discloses PET w/talc with good slipping and stacking properties with good transparency.
None of the prior art references disclose polyesters or polyester containers having a coefficient of friction of much less than 1.0, particularly polyesters or polyester containers having talc as an antiblock agent. There is, therefore, a need for new and improved polyesters and polyester containers having a reduced COF, particularly high clarity (low haze) containers that have a reduced COF.